Wireless data communication channels are affected by interference and various other effects. If not addressed, these effects can make wireless data communication channels unreliable. Some of these effects arise and/or are particularly difficult to address where one or more stations on the data communication channel are not at fixed locations. Time variations in a communication channel may be caused by moving terminals, moving interference sources, time-varying interference, and/or changing propagation paths caused, for example, by moving radiowave scatterers and the like. Examples of moving radiowave scatterers include close-proximity moving vehicles or scintillating scattering from a rippling water surface. An example of a time-varying interference signal is the signal from a frequency hopping communications system.
Sources of problems with wireless communication channels include effects that cause a low signal power relative to the noise power plus interference power, at the receiver. These include:                path loss caused by long distances or by other excessive radiowave attenuation between the transmitter and receiver terminals;        multipath fades, which result from destructive interference at a receiving antenna as a result of radiowave signals traveling via multiple propagation paths;        other signals competing to use the same radio spectrum at the same time (these signals are called interference). In licensed frequency bands, interference is typically less of a problem than it is within the unlicensed bands.        
Interference can be exacerbated when the path loss is large as can occur, for example, where data transmission occurs over long distances and/or where the directional transmit power of the transmitted signal is low. Directional transmit power is typically constrained by law (for example by the Radio Regulations). There are no legal constraints on the directional gain of a receiving antenna. However, the higher the directional gain of an antenna, the narrower its beamwidth. A narrow beamwidth makes it more difficult to achieve and maintain alignment between a receive antenna and a desired signal. High directional gain, fixed-beam antennas, are normally unsuitable for mobile terminals because of this beam alignment problem.
The above-mentioned problems can cause reduced data throughput in the communications link. In extreme cases the communication channel becomes unusable. Such extreme cases are ubiquitous in the unlicensed industrial, scientific, and medical (ISM) radio frequency bands, where many users exploit the freely available radio spectrum for communications, including attempts at long distance communications.
Many industrial and commercial links operate in non-licensed bands, for example, the industrial, scientific, and medical (ISM) bands. The ISM bands are defined by the Radio Regulations of the International Telecommunications Union Radiocommunication sector (ITU-R). The ISM bands include bands at about 0.9 GHz, 2.4 GHz, and 5.8 GHz, among others. Current commercial wireless systems use the ISM bands at frequencies as high as about 60 GHz. The advantage of using these non-licensed bands is that the spectrum is free to use. The use of other (licensed) spectrum is expensive.
OFDM -based WiFi (an 802-11 standard) has emerged as the de-facto standard technology for unlicensed broadband communications. The basic reason for its success is that the architecture, when mass produced, is very inexpensive. A single digital transmitter and receiver are used for many different data streams which occupy separate, adjacent narrow bandwidths. The use of the many data streams permits high data throughput. The many narrow bandwidth data streams sum to occupy a wide bandwidth. Unlike many wideband signaling systems, currently available OFDM systems are highly susceptible to narrowband interference signals. 802.11n, which is a current iteration of WiFi standards has various features that can be used to improve performance (e.g. to improve data throughput in environments where received signals have a low signal-to-noise ratio (SNR) per unit bandwidth). However, our measurements, as well as those of others have indicated that systems which use the 802.11 standards do not work well and often not at all, in the presence of interference.
In ISM bands, the maximum transmit power levels are restricted (regulated in North America by the FCC and Industry Canada), but there is no regulatory coordination in the sharing of this spectrum. Therefore, broadband OFDM communications systems operating in ISM bands are particularly susceptible to interference from other users of the spectrum.
Much of the energy in the 2.4 GHz and 5.8 GHz ISM bands is generated by WiFi devices. Although WiFi systems are designed to coexist in the same space and use the same spectrum, they still compete for available capacity. The problem of interference caused by other WiFi systems is increasing because the number of WiFi users is increasing. In some locations, there are so many competing signals (both from WiFi and non-WiFi signal sources) that high throughput systems, as required in several industrial/commercial links are unworkable using currently-available technology.
Industrial/commercial applications that deploy long-distance broadband links in ISM bands typically use commercial grade WiFi radio modules such as those available from Ubiquiti Networks of Milipitas Calif. USA coupled with high-gain, fixed antennas to achieve the required range. However, such fixed-antenna systems remain susceptible to interference.
It is known to use adaptive antennas for interference cancellation and for enhancing gain of wanted signals. An adaptive antenna ideally strives to maximize the received power of a wanted signal to the sum of interference power and noise power. This ratio is known as the SINR. Here and in the following discussion, “power”refers to the power within time- and frequency-bands of interest except as otherwise indicated. Adaptive antennas of various types can provide improved gain for receiving wanted signals and reduced gain for receiving unwanted signals. However, many adaptive antennas do not perform as well as would be desired, especially in the presence of interference.
An adaptive antenna, in receiving mode, normally comprises several antenna elements. Signals received at the different antenna elements are combined in order to optimize some aspect of transmission. Some adaptive antennas receive signals from different antenna elements antennas, and combine these signals using variable weights in order to get improved reception. In transmit mode, similar adjustments can be made to the antenna parameters, for example adjusting the weights of signals transmitted by different elements in order to maximize transmit gain in particular directions. Adaptive antennas at transmitting and receiving ends of a communication link can be jointly optimized to maximize transmission gain as well. This is a form of MIMO (multiple input, multiple output) communication.
U.S. Pat. No. 7,257,425 discloses a ‘smart antenna’ module which weighs and combines signals received by multiple antennas.
There is a need for robust data communication systems that are practical and cost-effective. There is a specific need for such systems suitable for providing ship-to-shore data communication or other data communication with a moving end-point. Among the many specific applications scenarios, there is a need for such systems suitable for providing ship-to-shore data communication or other data communication with moving terminals. Advantageously, such systems could operate in an ISM band. Ship-to-shore radio has finite range because of the severe path loss (for long ranges, the path gain can be proportional to the inverse-fourth power with distance, as opposed to inverse square law as in free space) over the water. Moreover, particularly with ferry services linking built-up ports, the port and its surrounding areas (often within, or close to, a city) feature many radio users, so the level of interference is high at the locations where one may wish to locate terrestrial transceivers.